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hpack, Unbounded variable integer decoding, CVE-2026-59980 (Moderate)
CVE-2026-59980 is a denial-of-service vulnerability affecting the python-hyper/hpack library, an HTTP/2 header compression implementation used as a downstream dependency by the python-hyper/h2 library. The flaw resides in the variable integer decoding routine, which is responsible for parsing HPACK-encoded integers that may use continuation bytes. When the decoder encounters a long sequence of 0xFF bytes, it enters an unbounded parsing loop that consumes quadratic CPU time relative to the input length, resulting in O(n^2) runtime complexity. An attacker can exploit this by sending a crafted HTTP/2 header block containing an excessive run of 0xFF bytes to a service that uses hpack for header decoding. Because the decoder does not restrict the length of the variable integer representation, the computation runs away, blocking further processing and effectively causing a denial-of-service condition. The vulnerability was assigned CWE-400 (Uncontrolled Resource Consumption) and carries a CVSS 4.0 base score of 6.3, indicating moderate severity. The issue affects versions prior to 4.2.0. A patch is available in python-hyper/hpack v4.2.0, which restricts variable integer decoding to uint32 to prevent run-away computation. As a workaround, input to the hpack decoder should be sanitized to filter out long sequences of 0xFF values before parsing.
DailyCVE Form:
Platform: python-hyper/hpack
Version: < 4.2.0
Vulnerability : Unbounded integer
Severity: Moderate
date: 2026-06-23
Prediction: 2026-09-24
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Bash Commands and Codes
Check installed hpack version
pip show hpack
Upgrade to patched version
pip install --upgrade hpack>=4.2.0
Sanitize input example (Python)
def sanitize_hpack_input(data: bytes) -> bytes:
Remove runs of 0xFF longer than 16 bytes
return re.sub(rb'\xff{17,}', b'', data)
Exploit: (Educational Purposes!)
To demonstrate the vulnerability in a controlled environment, a malicious HTTP/2 header block containing a long sequence of 0xFF bytes can be passed to the hpack decoder. The following Python snippet illustrates the trigger condition:
import hpack Crafted payload with 100,000 bytes of 0xFF malicious_payload = b'\xff' 100_000 decoder = hpack.Decoder() This call will consume excessive CPU time decoder.decode(malicious_payload)
Running this code on a vulnerable version causes the decoder to execute an O(n^2) integer parsing loop, effectively hanging the process.
Protection: from this CVE
- Upgrade to python-hyper/hpack version 4.2.0 or later, which restricts variable integer decoding to uint32.
- If upgrading is not immediately possible, implement input sanitization that filters out long sequences of 0xFF bytes before passing data to the hpack decoder.
- Monitor HTTP/2 header blocks for unusual patterns, such as excessive continuation bytes, and reject suspicious payloads at the application or network layer.
Impact:
A successful exploitation results in a denial-of-service condition. The affected service or application becomes unresponsive due to excessive CPU consumption, blocking legitimate requests and potentially causing a complete outage. The vulnerability does not lead to data confidentiality or integrity loss, but it can severely degrade availability in any environment where the python-hyper/hpack library is used to decode untrusted HTTP/2 headers.
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Sources:
Reported By: github.com
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